THEORETICAL-MODELS FOR CALCULATING THE TEMPERATURE RISE PRODUCED BY ULTRASOUND IN TISSUE, AND THE EXTENT TO WHICH THEY HAVE BEEN VALIDATED

There are a number of theoretical models aimed at describing temperature elevations in-vivo. The equations used have two major components, namely the energy source and the mechanism for cooling. The uncertainty involved in establishing these terms determines the accuracy with which we can predict the temperature elevation. Tissue cooling comes from two main sources: thermal conduction and perfusion. Thermal conduction is well understood theoretically and its contribution to cooling may be accurately modelled. Tissue perfusion is highly variable and difficult to model. This limits the accuracy with which the cooling function can be determined in cases where perfusion is an important factor. In clinical situations where narrow beams are used, and heated volumes are small (as can occur at the beam focus, or at bone surfaces) perfusion is unlikely to be dominant factor, especially where short dwell times are used. It is only useful to be able to estimate tissue temperatures using theoretical methods if one can have confidence in the values obtained. In an application such as that of the fetal exposure to ultrasound, the margin for error is small (a 1.5-degrees-C temperature rise may be acceptable, 2.0-degrees-C may not be). It is therefore important that tissue temperature estimation is sufficiently precise to exclude the possibility of unacceptably high temperatures occurring at any sensitive site. Currently, insufficient experiments have been carried out to verify the applicability of existing models to pulsed diagnostic fields, especially when there is bone within the field, variable but unquantified blood flow, or where there is a significant amount of non-linearity in the beam. Until further work on refining existing models has been carried out, prediction of safety from thermal modelling should be treated with some caution. Given the current state of sophistication of theoretical models, the most acceptable application lies in the determination of "worst case" temperature rises that may result from clinical ultrasound exposures.